The present invention relates to a magnetic resonance imaging (MRI) apparatus, a magnetic resonance imaging method, a scan apparatus, a program and a storage medium. The present invention relates particularly to a magnetic resonance imaging apparatus, a magnetic resonance imaging method, a scan apparatus, a program and a storage medium wherein a scan for, after the elapse of an idling time during which RF (Radio Frequency) pulses are repeatedly transmitted every time of repetition (TR) in such a manner that spins in an imaging area of a subject are respectively brought into an SSFP (Steady State Free Precession) state in a static magnetic field space, repeatedly transmitting the RF pulses every time of repetition to the imaging area in which each of the spins is brought into the SSFP state during the idling time, and receiving magnetic resonance signals generated at the imaging area, is executed plural times by a phase cycling method, and images about the imaging area are generated based on the magnetic resonance signals received by executing the scan plural times.
A magnetic resonance imaging apparatus excites spins in an imaging area of a subject in a static magnetic field space by a nuclear magnetic resonance (NMR) phenomenon, and generates images about the imaging area of the subject, based on magnetic resonance (MR) signals generated with the excitation thereof.
The magnetic resonance imaging apparatus has been used in various fields such as a medical field, an industrial field, etc. The imaging area of the subject has been photographed by various imaging or photographing methods according to photographic purposes. For example, as an imaging method used in the magnetic resonance imaging apparatus, there has been known an SSFP pulse sequence called an FISP (Fast Imaging with Steady-state Precession) or an FIESTA (Fast Imaging Employing Steady state Precession) (refer to, for example, a patent document 1).
In the SSFP pulse sequence, RF pulses are repeatedly transmitted to the imaging area of the subject in TR shorter than both of a vertical relaxation time and a horizontal relaxation time, and spins in the imaging area are respectively brought into an SSFP state. After magnetic resonance signals generated in the SSFP state have been received, images about the imaging area of the subject are generated based on the magnetic resonance signals. Here, respective gradient magnetic fields are applied in such a manner that the time integral values of the gradient magnetic fields applied in a slice selection direction, a phase encode direction and a frequency encode direction respectively are brought to zero within TR. That is, the phase shifted by each gradient magnetic field is reset by rewinding horizontal magnetization after acquisition of the magnetic resonance signals. Therefore, since the magnetic resonance signals including a FID (Free Induction Decay) signal and an echo signal are acquired in the present imaging method, signal strength is increased and hence the photography of each image having high contrast at high speed can be realized.
In the SSFP pulse sequence, however, band artifacts occur in the images due to the influence of magnetic-field nonuniformity and hence image quality might be deteriorated.
Therefore, there has been proposed a phase cycling method to suppress the occurrence of the band artifacts in the images (refer to, for example, a patent document 2).
[Patent Document 1] Japanese Unexamined Patent Publication No. 2003-10148
[Patent Document 2] Japanese Unexamined Patent Publication No. 2006-122222
FIG. 5 is a diagram showing phase-increased angles of transmitted RF pulses at scans executed by a phase cycling method. FIG. 5(a) shows the case of 2Nex, FIG. 5(b) shows the case of 3Nex, and FIG. 5(c) shows the case of 4Nex.
In the phase cycling method, as shown in FIG. 5, the angle (360°) of one circle is divided by the added number of times (Nex) to determine each phase-increased angle. The phase-increased angle is changed and the scan is executed plural times to obtain images at respective scans. Thereafter, the plural images are combined together to produce a combined image.
Described specifically, in the case of 2Nex, a first scan is first executed assuming that the phase-increased angle corresponding to the angle of each phase increased every TR is 0° as shown in FIG. 5(a). That is, RF pulses are repeatedly transmitted in such a manner that the phase of the RF pulse at each TR is sequentially transitioned or shifted to 0°, 0°, . . . to execute the first scan. Next, a second scan is executed assuming that the phase-increased angle is 180°. That is, RF pulses are repeatedly transmitted in such a manner that the phase of the RF pulse at each TR is sequentially transitioned to 0°, 180°, 360° (0°), 540° (180°), . . . to execute the second scan. Then, an image generated based on a magnetic resonance signal obtained by execution of the first scan and an image generated based on a magnetic resonance signal obtained by execution of the second scan are combined together.
In the case of 3Nex, RF pulses are first repeatedly transmitted in such a manner that the phase of the RF pulse at each TR is sequentially transitioned to 0°, 0°, . . . with the phase-increased angle as 0° as shown in FIG. 5(b), thereby to execute a first scan. Next, RF pulses are repeatedly transmitted in such a manner that the phase of the RF pulse at each TR is sequentially transitioned to 0°, 120°, 240°, 360°, . . . with the phase-increased angle as 120°, thereby to execute a second scan. Next, RF pulses are repeatedly transmitted in such a manner that the phase of the RF pulse at each TR is sequentially transitioned to 0°, 240°, 480° (120°), 720° (0°), . . . with the phase-increased angle as 240°, thereby executing a third scan. Thereafter, images generated based on magnetic resonance signals obtained by execution of the first, second and third scans are combined together.
In the case of 4Nex, RF pulses are first repeatedly transmitted in such a manner that the phase of the RF pulse at each TR is sequentially transitioned to 0°, 0°, . . . with the phase-increased angle as 0° as shown in FIG. 5(c), thereby to execute a first scan. Next, RF pulses are repeatedly transmitted in such a manner that the phase of the RF pulse at each TR is sequentially transitioned to 0°, 90°, 180°, 270°, 360°, . . . with the phase-increased angle as 90°, thereby to execute a second scan. Next, RF pulses are repeatedly transmitted in such a manner that the phase of the RF pulse at each TR is sequentially transitioned to 0°, 180°, 360° (0°), 540° (180°), 720° (0°), . . . with the phase-increased angle as 180°, thereby executing a third scan. Next, RF pulses are repeatedly transmitted in such a manner that the phase of the RF pulse at each TR is sequentially transitioned to 0°, 270°, 540° (180°), . . . with the phase-increased angle as 270°, thereby executing a fourth scan. Thereafter, images generated based on magnetic resonance signals obtained by execution of the first, second, third and fourth scans are combined together.
In the respective images generated by the scans executed so as to correspond to the added number of times (Nex) at the phase cycling method as described above, the positions where the band artifacts occur are shifted from one another. Therefore, the occurrence of the band artifacts is suppressed at the combined image generated by combining the respective images together, and hence excellent image quality is realized.
However, when the respective scans are executed in the present imaging method, the phase of the RF pulse is changed in a manner similar to the above during the idling time prior to the acquisition of the magnetic resonance signals and the RF pulses are repeatedly transmitted every TR, in order to acquire or collect the magnetic resonance signals in the SSFP state in which each stable signal is obtained. Therefore, there was a case where imaging or photographing time would reach a long period of time. In the case of, for example, 2Nex, the RF pulses are repeatedly transmitted in such a manner that the phase of the RF pulse at each TR is sequentially transitioned to 0°, 180°, 360° (0°), 540° (180°), . . . with the phase-increased angle of the RF pulse as 180° during the idling time at the execution of the second scan, thereby forming the SSFP states. In order to form the SSFP states, there is a need to idle-shot many RF pulses during the idling time. Therefore, the present imaging method might encounter difficulties in executing imaging or photography efficiently.